In a conventional multi-coin validator, coins pass along a path past a number of spaced sensor coils which are each energised to produce an inductive coupling with the coin. The degree of interaction between the coin and the coil is a function of the relative size of the coin and coil, the material from which the coin is made and also its surface characteristics. Thus, by monitoring the change in impedance presented by each coil, as the coin passes it, data indicative of the coin under test can be provided. The data can be compared with information stored in a memory to determine coin denomination and authenticity.
The geometry of the coils in relation to the coin to be tested, strongly influences the degree of interaction between the coin and the coil. By selecting different coil geometries for the coil, different interactions and hence different characteristics of the coin can be tested.
For example, UK Patent No. 2 169 429 in the name of Coin Controls Limited discloses coin discrimination apparatus utilising three inductive sensor coils, two of which are disposed to one side of the coin path and are of different diameters, together with a third coil which is arranged to wrap around the path so that the coin under test passes axially through it.